(1) Field of the Invention
The invention relates to a separable blade attachment for a bearingless rotor of a helicopter, including a lift-generating airfoil blade, a flexbeam enclosed by a control cuff or torque tube, and connection means forming a separable or releasable junction between the airfoil blade and the flexbeam and the control cuff. The flexbeam and the control cuff can be connected or not.
(2) Description of Related Art
During operation, the rotor blades are deflected in various directions and are thereby subjected to high loads in these various directions. The rotor blades must be designed to withstand these high loads while still providing the required flexibility or articulation to allow the blades to carry out flapping, lead-lag oscillating, and pitch angle variation movements.
Typically, a rotor blade of a bearingless rotor includes a structural element known as a flexbeam at the inner end of the blade connected to the rotor head. The flexbeam supports and transmits the centrifugal forces of the blade into the rotor head. Additionally, the flexbeam includes at least portions or regions that are flexurally and torsionally soft or flexible to allow the blade to undergo the above mentioned movements in a flapping direction, a lead-lag direction, and in a pitch angle direction. The torsionally soft portion of the flexbeam is arranged within a torsionally stiff control cuff or torque tube, through which the pitch angle control movements are transmitted to the lift-generating airfoil blade portion of the rotor blade. The airfoil blade typically extends from the outboard end of the control cuff to the outermost end of the rotor blade, i.e. the blade tip.
The vibrations of the rotor blades, and particularly the oscillations in the lead-lag direction, must be damped by appropriate damping elements. The damping effectiveness of the damping elements is predominantly dependent on the effective transmission of the lead-lag oscillating movements of the airfoil blade into the damping elements. Any softness or lack of force transmission through the blade/cuff attachment to the damping element will reduce the total resulting damping effect.
In order to allow the flexbeam/cuff unit and/or the airfoil blade to be separately manufactured and/or replaced in the event of damage, or in order to allow the airfoil blade to be pivoted and folded relative to the flexbeam/cuff unit, it is desired to provide a separable or releasable junction between the flexbeam/cuff unit and the lift-generating airfoil blade.
The separable or releasable junction is subject to high technical and mechanical demands, because it must reliably carry and transmit the substantially high centrifugal forces resulting during rotation of the rotor blade and all bending moments from flapping and lead lag movements of the rotor blade. The rotor blade has a longitudinal main load axis next to 25% of the average airfoil chord of the blade profile, said main load axis being essentially covered by the pitch axis of said rotor blade. At the level of the flexbeam said longitudinal main load axis next to 25% of the average airfoil chord of the blade profile corresponds to a longitudinal middle axis of the flexbeam.
The document US2008101934 A1 discloses an assembly for providing flexure to a blade of a rotary blade system including an upper support plate having an upper curved surface, a lower support plate having a lower curved surface, and a yoke positioned there between. An embodiment includes an assembly for providing flexure to a blade of a rotary blade system, including, an upper support plate having an upper curved surface, a lower support plate having a lower curved surface, and a yoke positioned there between and directly contacting the support plates wherein one of the curved surfaces is a non-circular arc that does not form part of the circumference of a circle.
The document EP 0 315 962 A2 discloses a helicopter rotor blade supported by a flexbeam to be rotatable about an axis of rotation, in which a pitching motion thereof is allowable. The rotor blade is provided with a device for changing the pitch and damping the lead-lag motion thereof. The device comprises a bushing in a hole formed at the inboard end of the flexbeam, an elastomeric pivot loosely fitted in the bushing, elastomeric dampers of cylindrical shape mounted on the upper and lower surfaces of the flexbeam and coupled with the upper and lower ends of the elastomeric pivot by means of nuts, and torque arms extending through the bushing and the elastomeric dampers and having the central portion thereof connected to central shafts of the elastomeric pivots. Each of the pitch sleeves has both ends outwardly projecting beyond the elastomeric dampers and secured to a pitch sleeve which encloses the flexbeam. Therefore, the relative position between the pitch sleeve and the elastomeric pivot does not change even when lead-lag motion is imparted to the rotor blade.
The document U.S. Pat. No. 4,427,340 A discloses helicopter rotors and more particularly rotor mounting involving a composite fiber-reinforced unitary yoke with resilient inplane restraints.
The document U.S. Pat. No. 6,126,398 A discloses a rotor blade for a bearingless rotor of a helicopter with a lift-generating airfoil blade, a flexbeam connecting the airfoil blade to a rotor head, and a control cuff enclosing the flexbeam. The junction between the flexbeam and the airfoil blade is a separable junction to allow the airfoil blade to be folded in a simple manner while maintaining a high lead-lag stiffness and reduced structural height of the junction. The junction is formed by two connection arms arranged side-by-side in the lead-lag plane of the rotor blade.